Phase transition method of organic based complex and its functional element

ABSTRACT

A method for performing a phase transition of an organic complex in which the phase transition can be performed with a high performance under specific conditions and a functional element using the same are provided. When EDO-TTF-based complex crystals are irradiated with photons as weak as about 1 to about 10 μJ per square centimeter, the EDO-TTF-based complex crystals undergo a phase transition to a metal phase (high temperature phase) and an insulator phase (low temperature phase), thereby changing the reflection spectrum and the electric conductivity. Thus, the operation is performed in a wavelength region of 1.5 to 0.8 μm with a high-sensitivity, at a high speed, and at room temperature.

TECHNICAL FIELD

The present invention relates to a method for performing a phasetransition of an organic complex and a functional element using thesame, and in particular, to a method for performing a phase transitionof an EDO-TTF-based complex and a functional element using the same.

BACKGROUND ART

Although the above EDO-TTF-based complex crystals are not used, thefollowing Patent Documents 1 to 5 disclose liquid crystal materials ordevices serving as an optical switch or performing a phase transition.

[Patent Document 1] Japanese Unexamined Patent Application PublicationNo. 5-53088

[Patent Document 2] Japanese Unexamined Patent Application PublicationNo. 5-262698

[Patent Document 3] Japanese Unexamined Patent Application PublicationNo. 6-116246

[Patent Document 4] Japanese Unexamined Patent Application PublicationNo. 6-239786

[Patent Document 5] Japanese Unexamined Patent Application PublicationNo. 8-92258

DISCLOSURE OF INVENTION

The present inventors have been studying the changes in reflectionspectrum and electric conductivity in EDO-TTF-based complex crystals andhave found significant changes in reflection spectrum and electricconductivity in the EDO-TTF-based complex crystals under specificconditions, thus creating an efficient functional element.

In view of the above situation, it is an object of the present inventionto provide a method for performing a phase transition of an organiccomplex in which the phase transition can be performed with a highperformance under specific conditions, and a functional element usingthe same.

In order to achieve the above object, the present invention provides thefollowing:

[1] A method for performing a phase transition of organic complexcrystals including a step of changing reflection spectrum and electricconductivity using EDO-TTF-based complex crystals with a single photonper 2,000 to 5,000 molecules.

[2] The method for performing a phase transition of organic complexcrystals according to Item [1], wherein the change in reflectionspectrum significantly occurs in a wavelength region of 1.5 to 0.8 μm.

[3] The method for performing a phase transition of organic complexcrystals according to Item [2], wherein the rate of change in reflectionspectrum is 1 to 100 ps.

[4] The method for performing a phase transition of organic complexcrystals according to Item [2], wherein the change in reflectionspectrum is 100%.

[5] The method for performing a phase transition of organic complexcrystals according to any one of Items [1] to [4], wherein a high-speedoptical switching is performed at room temperature range and in aterahertz region.

[6] The method for performing a phase transition of organic complexcrystals according to Item [1], wherein resistivity and magneticsusceptibility are suddenly changed significantly by performing thephase transition at a temperature of about 280K, thereby sensing thechanges in resistance and magnetism.

[7] A functional element including organic complex crystals using themethod for performing a phase transition of organic complex crystalsaccording to any one of Items to [6].

[8] The functional element including organic complex crystals accordingto Item [7], wherein the functional element can operate in a wavelengthregion of 1.5 to 0.8 μm with a high sensitivity, at a high speed, and atroom temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a functional element including organiccomplex crystals, which shows an embodiment of the present invention.

FIG. 2 is a schematic view of a system for measuring characteristics ofthe change in reflectance of the functional element according to thepresent invention using fixed light.

FIG. 3 is a graph showing measurement results of the change inreflectance of the functional element according to the present inventionusing fixed light.

FIG. 4 is a graph showing measurement results of the change inreflectance of the functional element according to the present inventionusing fixed light.

FIG. 5 includes views showing a quasi-one-dimensional organic conductorof ¼-filled (EDO-TTF)₂PF₆.

FIG. 6 shows a chemical structure of EDO-TTF.

FIG. 7 is a graph showing an optical pumping effect of (EDO-TTF)₂PF₆crystals in a low temperature phase (T=270K)

FIG. 8 is a graph showing a temperature dependence of the change inreflectance when 270K is defined as a standard temperature.

FIG. 9 is a graph showing an optical pumping effect of (EDO-TTF)₂PF₆crystals in a high temperature phase (T=290K), which shows a comparativeexample.

FIG. 10 is a graph showing a polarization dependence of pump light in(EDO-TTF)₂PF₆ crystals, which shows a comparative example.

FIG. 11 is a graph showing a dependence of pump light intensity in(EDO-TTF)₂PF₆ crystals.

FIG. 12 includes graphs showing characteristics of a sensor showing asecond embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention can provide the following advantages:

(1) In EDO-TTF-based complex crystals, the electric conductivity and thereflection spectrum can be changed with a single photon per 2,000 to5,000 molecules.

(2) The change in reflection spectrum can overlap the frequency regionthat can be used in optical communication.

(3) The rate of the change in reflection spectrum is high and is 1 to100 ps.

(4) The rate of the change in reflection spectrum can be achieved atroom temperature.

(5) Since EDO-TTF-based complex crystals are an organic substance, thecrystals can be readily grown and produced.

(6) In a known method, the change occurs at about liquid nitrogentemperature and the rate of the change is several tens to 100 ps. Inaddition, the change in reflection spectrum at a wavelength region usedin communication is several tens of percent. In contrast, the presentinvention provides a rate of change of several hundreds of percent and adramatic high-speed change.

According to the present invention, when 2,000 to 5,000 molecules of anEDO-TTF-based complex crystal is irradiated with a single photon[photons as weak as about 1 to about 10 μJ per square centimeter], theEDO-TTF-based complex undergoes a phase transition to a metal phase(high temperature phase) and an insulator phase (low temperature phase),thereby changing the reflection spectrum and the electric conductivity.Furthermore, the change in reflection spectrum is significantly large ina wavelength region of 1.5 to 0.8 μm, which is important incommunication. Also, the change is completed at a rate of 1 to 2 ps andcan be performed at room temperature range. Thus, the present inventorshave confirmed the possibility of the crystals serving as a high-speedoptical switching element in a terahertz region.

In addition, the phase transition due to the temperature occurs at about280K wherein the resistivity and the magnetic susceptibility are alsodrastically changed. These phenomena indicate that the EDO-TTF-basedcomplex can function as a sensor.

Embodiments of the present invention will now be described in detail.

First Embodiment

FIG. 1 is a schematic view of a functional element including organiccomplex crystals, which shows an embodiment of the present invention.

In the figure, reference numeral 1 indicates a substrate, referencenumeral 2 indicates organic complex crystals [EDO-TTF-based complex,(EDO-TTF)₂PF₆] provided on the substrate 1, reference numeral 3indicates a transparent electrode, reference numeral 4 indicates anirradiation source of photons, and reference numeral 5 indicates photonsirradiated from the irradiation source 4 of photons.

Firstly, an optical pumping effect in the metal-insulator transition of(EDO-TTF)₂PF₆ will now be described.

FIG. 2 is a schematic view of a system for measuring characteristics ofthe change in reflectance of the functional element according to thepresent invention using fixed light.

In the figure, reference numeral 10 indicates a functional element ofthe present invention, reference numeral 11 indicates pump light,reference numeral 12 indicates probe light, reference numeral 13indicates a spectroscope, and reference numeral 14 indicates an opticalreceiver.

Subsequently, the temperature dependence of reflectance in (EDO-TTF)₂PF₆will now be described.

FIG. 3 is a graph showing measurement results of the change inreflectance of the functional element according to the present inventionusing fixed light. FIG. 3 corresponds to the area indicated by symbol Din FIG. 4, which will be described below.

In the figure, the pump light had an energy of 1.55 eV (800 nm) and theprobe light had an energy of 1.38 eV (900 nm). Polarized light (E//b)parallel to the laminated direction of the organic complex crystals wasused as both the pump light and the probe light. In other words, lighthaving a wavelength of 800 nm (1.55 eV), which approximately correspondsto the charge-transfer transition energy (1.38 eV) required forF⁺F⁺→F²⁺F⁰, was selected as the pump light.

In the figure, Line A represents the case at 250K, Line B represents thecase at 260K, Line C represents the case at 270K, Line D represents thecase at 280K, and Line E represents the case at 290K.

As shown in the figure, the reflection spectrum is significantly changedat the threshold of the metal-insulator transition.

FIG. 4 is a graph showing measurement results of the change inreflectance of the functional element according to the present inventionusing fixed light.

FIG. 4 is a graph showing the characteristics of reflectance measured asa function of the wavenumber by excitation and probe spectroscopy. Atime-resolved observation by excitation and probe spectroscopy wasperformed to observe a photo-induced phase transition from thereflection spectrum. Symbol A indicates a high temperature phase (290K),symbol B indicates a low temperature phase (270K), and symbol Cindicates a wavelength region used in communication.

As is apparent from the figure, the reflection spectrum is significantlychanged at a threshold of the transition temperature (280K).

Furthermore, when the metal (high temperature) phase and the insulator(low temperature) phase are switched by irradiating light, a significantchange in reflectance (high-speed switch for communication) can beprovided in a wavelength region of 1.5 to 0.8 μm.

The crystal structure and the features of the (EDO-TTF)₂PF₆ crystalswill now be described.

FIG. 5 includes views showing a quasi-one-dimensional organic conductorof ¼-filled (EDO-TTF)₂PF₆. FIG. 5(a) shows the high temperature phase(300K) and FIG. 5(b) shows the low temperature phase (260K).

The quasi-one-dimensional organic conductor of ¼-filled (EDO-TTF)₂PF₆has the following properties.

(1) The organic conductor has +0.5 valence per donor. (2) The b-axis isdirected in the laminated direction. (3) In the low temperature phase inFIG. 5(b), the anion (PF₆) is regularly oriented to form a tetramer. Inaddition, a flat molecule (F) and a bending molecule (B) are arrayedperiodically. In other words, the structure is as follows:. . . -F-F-B)-(B-F-F-B)-(B- . . .

(4) According to the Raman spectra, the bending molecule (B) has acharge of zero and the flat molecule (F) has a charge of +1.

Substances in which the phase transition by optical pumping can becontrolled are promising as a next-generation optical element. In orderto achieve the practical application, the optical control must beperformed at about room temperature.

FIG. 6 shows a chemical structure of EDO-TTF.

Whether a photo-induced phase transition is performed or not in this(EDO-TTF)₂PF₆ will now be demonstrated.

Firstly, a metal-insulator transition of (EDO-TTF)₂PF₆ crystals will nowbe described.

In the low temperature phase,

(1) The crystals are in a [0110] type charge-ordered state.

That is, the crystals become as follows:

Herein, F represents a flat state and B represents a bending state.

(2) The donor forms a tetramer.

(3) The anion (PF₆) has a regular orientation.

Accordingly, the metal-insulator transition of (EDO-TTF)₂PF₆ crystals isa cooperative phenomenon of a charge-ordering transition, a Peierlstransition, and an anion ordering transition.

Subsequently, an optical pumping effect of (EDO-TTF)₂PF₆ crystals in thelow temperature phase will now be described.

FIG. 7 is a graph showing the optical pumping effect of (EDO-TTF)₂PF₆crystals in the low temperature phase. The abscissa indicates a delaytime (ps) and the ordinate indicates the reflectance (−ΔR/R). The pumplight had an energy of 1.55 eV (800 nm) and the probe light had anenergy of 1.38 eV (900 nm). Polarized light (E//b) parallel to thelaminated direction of the organic complex crystals was used as both thepump light and the probe light. The temperature T was 270K.

As is apparent from the figure, as the delay time was changed, thereflectance was changed. When the penetration length of the light is 10μm, 4,000 donors undergo the phase transition per photon at a pump lightintensity of 2×10¹⁴ photons/cm².

FIG. 8 is a graph showing a temperature dependence of the change inreflectance when 270K is defined as a standard temperature.

In the figure, the probe light was the polarized light (E//b) parallelto the laminated direction of the organic complex crystals and had anenergy of 1.38 eV.

According to this result, when the value of reflectance is about zero,the organic complex crystals is in the low temperature phase and whenthe value of reflectance is about 0.8, the organic complex crystals isin the high temperature phase. The transition temperature is 280K.

The change in reflectance −ΔR/R was calculated as follows:−ΔR/R=(R _(T) −R _(270K))/R _(270K)

R_(270K): reflectance at 270K

R_(T): reflectance at a temperature T during measuring

In the present invention, it was confirmed that even a single photoncould change 8,000 to 10,000 molecules of the donor.

In addition, the following can be described.

FIG. 9 is a graph showing an optical pumping effect of (EDO-TTF)₂PF₆crystals in the high temperature phase (T=290K), which shows acomparative example. The pump light was polarized light (E//b) parallelto the laminated direction of the organic complex crystals and had anenergy of 1.55 eV. The pump light intensity was 2.0×10¹⁴ photons/cm².The probe light was also the polarized light (E//b) and had an energy of1.38 eV. The abscissa indicates a delay time (ps) and the ordinateindicates the reflectance (−ΔR/R).

As is apparent from the figure, even when the delay time was changed,the phase transition from the high temperature phase to the lowtemperature phase did not occur. [In contrast, under a strong pump light(Line C) in the low temperature phase (T=270K), the photo-excited phasehas a long lifetime. Therefore, the subsequent excitation pulse reachesbefore the phase is returned to the former state.]

FIG. 10 is a graph showing a polarization dependence of pump light in(EDO-TTF)₂PF₆ crystals, which shows a comparative example. The pumplight had an energy of 1.55 eV. The pump light intensity was 2.0×10¹⁴photons/cm². The probe light had an energy of 1.38 eV. The temperature Twas 270K. The abscissa indicates a delay time (ps) and the ordinateindicates the reflectance (−ΔR/R). In the figure, Line A indicates thecase where the polarized light is parallel (E//b) to the laminateddirection of the organic complex crystals and Line B indicates the casewhere the polarized light is orthogonal (E™b) to the laminated directionof the organic complex crystals.

As is apparent from the figure, even when the polarization of the pumplight was changed, a significant difference was not observed.

FIG. 11 is a graph showing a dependence of pump light intensity in(EDO-TTF)₂PF₆ crystals. The pump light was polarized light (E//b)parallel to the laminated direction of the organic complex crystals andhad an energy of 1.55 eV. The pump light intensity was 2.0×10¹⁴photons/cm². The probe light was also the polarized light (E//b) and hadan energy of 1.38 eV. The temperature T was 270K. The abscissa indicatesa delay time (ps) and the ordinate indicates the reflectance (−ΔR/R). Inthe figure, Line A indicates 1.0×10¹⁴ photons/cm², Line B indicates2.0×10¹⁴ photons/cm², and Line C indicates 3.0×10¹⁴ photons/cm².

As is apparent from the figure, the pump light (Line A) whose intensityis lower than the pump light intensity (Line B) causing the phasetransition does not cause the phase transition. Under a strong pumplight (Line C) in the low temperature phase (T=270K), the photo-excitedphase has a long lifetime. Therefore, the subsequent excitation pulsereaches before the phase is returned to the former state.

Second Embodiment

Subsequently, a sensor element serving as a functional element andshowing a second embodiment of the present invention will now bedescribed.

FIG. 12 includes graphs showing characteristics of a sensor showing thesecond embodiment of the present invention. In FIG. 12(a), the abscissaindicates 1000/T (K⁻¹) and the ordinate indicates resistivity. In FIG.12(b), the abscissa indicates T/K and the ordinate indicates magneticsusceptibility.

It has been confirmed that (EDO-TTF)₂PF₆ crystals undergo ametal-insulator transition due to the temperature at a threshold of thetransition temperature (T_(MI)=280K)

As shown in FIG. 12(a), the resistivity is drastically changed at athreshold of T_(MI)=280K.

As shown in FIG. 12(b), the magnetic susceptibility shows a smallhysteresis at the transition point (first-order transition).

Thus, it is apparent that the phase transition due to the temperatureoccurs at about 280K wherein the resistivity and the magneticsusceptibility are drastically changed. This phenomenon shows that thiscomplex can function as a sensor.

The present invention is not limited to the above embodiments. Variousmodifications can be made based on the purpose of the present invention,and those modifications are not excluded from the scope of the presentinvention.

INDUSTRIAL APPLICABILITY

The functional element using the phase transition of an EDO-TTF-basedcomplex of the present invention is advantageous as an optical switchingelement in the communication field in the near future where a vastamount of information flies at a high speed because a significant changecan be achieved particularly in a wavelength region of 1.5 to 0.8 μm.

1. A method for performing a phase transition of organic complex crystals comprising a step of changing reflection spectrum and electric conductivity using EDO-TTF-based complex crystals with a single photon per 2,000 to 5,000 molecules.
 2. The method for performing a phase transition of organic complex crystals according to claim 1, wherein the change in reflection spectrum significantly occurs in a wavelength region of 1.5 to 0.8 μm.
 3. The method for performing a phase transition of organic complex crystals according to claim 2, wherein the rate of change in reflection spectrum is 1 to 100 ps.
 4. The method for performing a phase transition of organic complex crystals according to claim 2, wherein the change in reflection spectrum is 100%.
 5. The method for performing a phase transition of organic complex crystals according to any one of claims 1 to 4, wherein a high-speed optical switching is performed at room temperature range and in a terahertz region.
 6. The method for performing a phase transition of organic complex crystals according to claim 1, wherein resistivity and magnetic susceptibility are suddenly changed significantly by performing the phase transition at a temperature of about 280K, thereby sensing the changes in resistance and magnetism.
 7. A functional element comprising organic complex crystals using the method for performing a phase transition of organic complex crystals according to any one of claims 1 to
 6. 8. The functional element comprising organic complex crystals according to claim 7, wherein the functional element can operate in a wavelength region of 1.5 to 0.8 μm with a high sensitivity, at a high speed, and at room temperature. 